Westby, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 28 °F (low) to 50.8 °F (high). Annual precipitation averages 14.5 Inches. Westby is located in USDA Plant Hardiness Zone 3b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Sep.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -0.6 °F | 18.8 °F | 0.45 in. |
| February | 3.6 °F | 23.6 °F | 0.32 in. |
| March | 15.2 °F | 35.5 °F | 0.57 in. |
| April | 27.8 °F | 52.4 °F | 0.9 in. |
| May | 39.6 °F | 64.7 °F | 1.88 in. |
| June | 49.9 °F | 72.8 °F | 3.28 in. |
| July | 54.4 °F | 79.8 °F | 2.46 in. |
| August | 52.3 °F | 80.2 °F | 1.54 in. |
| September | 42.1 °F | 69.7 °F | 1.26 in. |
| October | 29.8 °F | 53.7 °F | 0.89 in. |
| November | 16.4 °F | 35.5 °F | 0.5 in. |
| December | 4.9 °F | 23.4 °F | 0.46 in. |
| Annual | 28 °F | 50.8 °F | 14.51 in. |
| Average First Frost Date | Sep.11 - Sep. 20 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 3b : -35°F to -30°F |
| Plant Hardiness Zone - 2012 | Zone 3b : -35°F to -30°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |